1. Field of the Invention
This invention relates to ZnO, and a method and apparatus for fabricating the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., Ref [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
There has recently been a surge of interest in zinc oxide (ZnO); but, in fact, the material has long been of importance for a wide variety of applications, from sunscreen and pigments, to rubber manufacturing and varistors. However, these historic applications of ZnO have usually only required powdered forms of ZnO, or the polycrystalline ceramics formed from consolidating and sintering ZnO powders. As such, the ZnO requirements of these applications have largely been met by the materials produced directly from Zn ores or metallic Zn in large scale, “French” or “American” type industrial processes. Much of the recent interest in ZnO stems from newly developed and yet to be developed potential applications in the electronics and renewable energy industries. Many of these emerging applications for ZnO will have more rigorous specifications on the form and quality of the ZnO used than typical historical applications. Rather than simple powder, many of these applications will require ZnO in the form of thin films, nanoparticles, single crystals, and epitaxial material. Current commercial methods for producing these advanced forms of ZnO often utilize high temperatures, high or low pressures, toxic and/or highly specialized chemicals, and complex equipment, all of which lead to a high cost of production. In addition, each of the different forms of ZnO used for advanced applications typically requires its own specialized method for production.
Bulk ZnO single crystals are typically produced using either the hydrothermal method or a melt based growth method. The melting point of ZnO is near 2000° C., but ZnO at these temperatures will decompose to Zn metal and oxygen at atmospheric pressure. Therefore, melt based methods for producing ZnO crystals require extremely high temperatures, as well as controlled atmosphere and/or pressure. Although the conditions used are less extreme, the hydrothermal method still requires heavy-duty autoclaves capable of withstanding the high temperatures (300-400° C.) and high pressures (80-100 MPa) used. The hydrothermal growth solutions are also extremely corrosive and these autoclaves must be lined with non-reactive materials like platinum. Compared to these methods, the low temperatures and atmospheric pressure used the current invention allow for less energy consumption, less expensive equipment, and a less hazardous process.
Industrially, ZnO thin films are often deposited by a physical vapor deposition method, like magnetron sputtering or pulsed laser deposition (PLD), but chemical vapor and chemical solution methods have also been explored. The major drawback of magnetron sputtering and PLD is the need to maintain the very low pressure growth environment needed to create the plasma which sputters material from the target. The creation and control of the plasma also require expensive equipment and significant amounts of power. Chemical vapor deposition techniques, such as metalorganic chemical vapor deposition (MOCVD) also require low pressure atmospheres and expensive equipment, as well as specialty gases and chemical precusors. The present invention is related to the aqueous solution deposition techniques of chemical bath deposition (CBD) and electrodeposition, which can also be used to produce ZnO films, but offers important advantages over these techniques. Like the current invention, both CBD and electrodeposition techniques produce ZnO from dissolved Zn complexes. However, electrodeposition is limited to conductive substrates. The CBD has more versatility for substrates, but has not been demonstrated for the deposition of epitaxial films. Typically, CBD also results in considerable amounts of precursors being wasted.
The current industrial uses for ZnO nanostructures and nanoparticles are fairly limited. However as utilization increases, the current method has potential to be more scalable than the other techniques for producing nanostructures and nanoparticles which are found in the academic literature on the subject. Methods found in the literature for producing ZnO nanostructures and particles include vapor techniques and both aqueous and non-aqueous solution techniques.
The current invention presents a low temperature aqueous method that could potentially be used to synthesize all of the forms of ZnO mentioned above. While this technique has aspects in common with the established aqueous solution techniques of hydrothermal crystal growth and chemical bath deposition, crucial aspects of the current invention give the method disclosed here important advantages over these prior art aqueous solution methods, as well as over non-solution based techniques.